Combining superior alleles from two homozygous populations in a cross-fertilizing species

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Abstract
SUMMARY: The issue of incorporating the good alleles from two homozygous populations of a cross-fertilizing species into a single improved population was investigated assuming independent assortment, no epistasis, and either complete dominance (of the favourable or the unfavourable alleles) or additive genes. The selection limit in the foundation stock is a function of the effective population size (N), the proportion (x) contributed by the better source population (P1), the difference in relative fitness between single locus homozygotes and the proportion of loci (that will segregate in the foundation stock) fixed favourably in P1. In real life the last two of these are never known. We therefore focused on the response limits given x = 0·5, 0·75, 0·875 or the optimum value of x (which is a function of the other three parameters). Our general finding was that in situations where N is large enough so that a major portion of the potential can be achieved, the F2 population should be used as the foundation stock when the two source populations do not differ greatly in performance; but when one population performs considerably better than the other, the first backcross (but not second backcross) would be the choice.